Preventing Pest and Disease Problems

You’ve worked hard to test, amend, and prepare your soil, carefully planting the right plants at the right time of year. You’ve mulched and rotated, and your garden looks great. The last thing you’d want to see happen is for your beautiful garden to be destroyed by insects or disease.

Happily, organic gardeners typically face fewer pest problems than chemical gardeners. Their gardens are healthier because they pay attention to the health of the soil and the species of plants within them. But even organic gardens face pest and disease problems sometimes. Wise gardeners do much to prevent these problems, and react quickly and with close aim when they occur.

The best way to solve a garden problem is never to allow it to occur. By planting resistant and native plants, using crop rotations, keeping your garden clean, and encouraging biological diversity in your garden, you can prevent many pest and disease problems from occurring.

Plant to Discourage Problems

Preventive planting is one of the best ways to keep pests and diseases at bay. The goal of preventive planting techniques is not to kill pests or disease, but to fill your garden in ways that won’t encourage bugs and blights.

  • Plant Disease-Resistant Varieties – Whenever possible, choose disease-resistant plant cultivars. Remember that no plant is completely resistant, and that even resistant plants only withstand the specific disease or diseases that are listed. If a certain disease is common in your area, though, resistant species are a must.
  • Plant Native Varieties – Native plants have often evolved their own defenses against predators that are common to an area. By planting native plant species, you are keeping the natural ecosystem in balance.
  • Rotate Crops – When you keep the same plant or members of the same plant family in the same area of your garden for several years, you provide a continual source of food for the pests that attack that crop family. When you rotate crops, you don’t give time for any particular pest to become a widespread problem.
  • Use Companion Planting – Companion planting is one of the oldest ways of preventing insect damage to food crops. Certain plants hide your crops or give off odors that deter insects; others provide breeding grounds for beneficial insects or provide an alternate food source for pest insects. Catnip, mints, and marigolds are all well known for their insect-fighting abilities, while nasturtiums draw aphids away from other plants.

  • Keep your Garden Clean

    If you leave food on the counter, you’ll have ants or roaches in your house. The same is true for your garden. Keeping your garden clean is an important step in preventing pest and disease problems.

    Remove Damaged and Diseased Plants – When plants have been damaged by disease or pests, prune off the damaged areas and remove them from the garden. One rust-ridden plant can easily turn into several if the fungus is allowed to spread; by removing all traces of pests and diseases, you’re quarantining the garden.

    Clean your Tools – Once per year, clean all of your garden tools with a 10% bleach solution to kill any disease-causing agents that may be clinging to them. If you’ve used any of your tools on a diseased plant, wash it right away with a bleach solution.

    Keep your Plants Intact – Any wound on a plant can allow diseases to enter. Prune with care, and don’t step on your plants or damage them with a weed eater.

    Encourage Birds and Beneficials

    The sound of birds chirping is more than just music to an organic gardener–it means that somebody is patrolling the garden, looking for the bugs that could damage the plants. Birds and beneficial insects are an important tool in an organic pest-management plan, and gardeners should invite them into the garden by providing food and a place to stay.

    Feed the Birds and Bugs – The trick for getting birds and beneficial insects to visit your garden is to feed them. Put up birdfeeders and keep them stocked through the winter, and they will build nests nearby. Include plants with small flowers, to provide plenty of pollen to beneficial insects. Some plants that beneficial insects are particularly fond of include: buckwheat, dill, fennel, spearmint, white clover, goldenrod, morning-glory, and yarrow.

    Don’t be Squeamish – Many of the animals that eat the most pest insects are the animals that people typically fear. Bats, snakes, toads, and spiders are all powerhouse insect controls. Learn to tell poisonous snakes and spiders from harmless ones, and leave the harmless ones alone. Build bat houses and hang them on trees, and leave broken ceramic pots upside-down in the garden to give toads a home.

    Trap or Bar the Bugs that Find Your Garden

    Even if you use all of the techniques mentioned in this or any other gardening book, pests will still find your garden. Cutworms, slugs, Japanese beetles, and other pests are remarkably resourceful, and they will find a way in. Before you start spraying, though, try some simple traps and barriers when you first spot these pests.

    Barriers – Barriers allow pests into your garden, but keep them away from the plants they feed on. Some simple barriers include:

    • Row covers – Covering fruits and vegetables with fine mesh will allow water and sunlight in, but will keep airborne insects out. They won’t be able to get to your strawberries or your broccoli, so you can keep your crop to yourself.
    • Bottle cloches – Cut the bottoms off of your used plastic soda bottles, and throw the caps away. Put the sawed-off top of the bottle over young plants to keep slugs and other crawling pests away.
    • Cutworm collars – To prevent cutworms from damaging your transplants, plant them inside of a cutworm collar. Cut a strip of cardboard and tape them into a circle about 5 cm tall. Press half of the collar into the ground around your plant to keep cutworms out.

    Traps – Rather than keeping pests away, traps lure them in. Glue, water, or walls prevents the pests from getting any farther than the trap, and keeps your garden safe.

    • Sticky tree bands – Sticky compound can be painted around the trunks of mature trees, or on a strip of plastic wrapped around saplings. Pests that don’t have wings, including gypsy moth caterpillars, will get stuck in the goo and won’t make it to the leaves.
    • Pheromone traps – Nurseries stock traps that are baited with sex pheromones that lure pests into a glue or water trap. They only work against the target bug, so know which pests are your problem.
    • Beer traps – One of the simplest ways to trap slugs is to dig a hole large enough for a small cup, and fill the cup with beer. Slugs are drawn to the scent, and drown.


    Using Botanical Poisons

    Some insecticides, known as botanical poisons, are acceptable for use in organic gardens. These poisons come from plant parts, and vary in the numbers of insects they kill and how quickly they work.

    Wear protective clothing, including a mask and gloves, when working with insecticides.

    Insecticide Safety – Even botanical poisons are often toxic to humans. Use the same safety precautions with organic insecticides as you would with chemical poisons. Keep them away from children, wear protective clothing, follow the label precautions, and wash up carefully when you’re finished.

    Least Harmful First – Encouraging biological diversity is one of the most important tenets of organic gardening. Target your insecticides as narrowly as possible to kill only the pest, not the millions of other creatures who live in your garden. Beneficial insects do far more work to kill pests than any spray can, and they work year-round, so protect them by using the least harmful insecticide that will get rid of your particular pest problem.

    Insecticide Source Uses and Notes
    Growth Regulators Chemical Mimics insect hormones and disrupt feeding, growth, or reproduction of target species
    Insecticidal oils Petroleum, plant oils Blocks an insects’ oxygen supply. May discolor or damage plants
    Diatomaceous earth Fossilized algae shells Sharp edges of these microscopic particles harm soft-bodied insects
    Insecticidal soaps Fatty-acid solutions Kills soft-bodied insects
    Neem oil Neem trees Broad-spectrum insecticide with low toxicity to mammals and beneficial insects
    Pyrethrins Pyrethrum daisy flowers Broad-spectrum insecticide; highly toxic to fish and moderately toxic to mammals
    Rotenone South American legumes Kills aphids, beetles, and whiteflies. Highly toxic to fish, birds, and pigs
    Ryania Shrub, Ryania Speciosa Paralyzes insects; good against codling moths, cabbage worms, Japanese beetles, Mexican been beetles
    Sabadilla Seeds of a South American plant Kills hard-to-kill insects as a last resorts; toxic to bees and humans
    Nicotine Tobacco plants Effective against soil pests and leaf-chewing insects; highly toxic to humans


    Using Fungicides

    Organic fungicides are available to prevent a wide variety of plant diseases. Good gardening practices are the best way to prevent plant diseases, but a fungicide regime can help stop the spread of a persistent disease that has invaded your garden. Organic fungicides only prevent disease, they don’t eradicate it, so don’t expect them to cure your sick plants.

    Like insecticides, many fungicides are toxic to humans. Use caution when spraying fungicides, and follow package directions for store-bought fungicides.


    Fungicide Uses Notes
    Microbial fungicides Prevent diseases from becoming established Often painted on tree wounds to prevent decay
    Horticultural oil Prevents rust and mildew Don’t use on plants that are heat- or drought-stressed
    Garlic spray Fungicide and insecticide Blend 1 liter of water with 7-10 cloves; strain and spray on plants
    Compost tea Prevents back spot and blight Mix water and compost equally and let stand for 10 days; strain and spray on plants
    Baking soda Prevents fungal spores Even more effective when combined with oil
    Sulfur Prevents diseases from developing Damages beneficial insects and soil microorganisms; moderately toxic to mammals and humans
    Copper Powerful fungicide; kills most disease organisms Harmful to soil and beneficial insects; can damage plants or stunt growth; highly toxic to humans